This invention relates to an apparatus for sensing the relative motion of a second object some distance from the reference sensor without the need of transmitted excitation such as electromagnetic or sonic radiation. In particular, this invention may be used for the rapid measurement of the ground speed of an aircraft while flying at various altitudes.
It is often advantageous for an aircraft to be able to determine rapidly and accurately its ground velocity, as opposed to its velocity with respect to the air. For example, the knowledge of accurate ground speed during the approach and landing phases of the aircraft's operation can provide much useful information for aircraft safety. Knowledge of true ground velocity can improve the proper initialization and operation of the wheel brake anti-skid control system by providing necessary data to the hydroplaning and touchdown spin-up logic of the control system. Also, a continuous comparison between the indicated air speed and true ground speed gives an accurate measurement of the wind component along the axis of the aircraft. This information is essential for determining potential wind shear problems that the aircraft may encounter during approach and landing, and allows the proper correction of the characteristics procedure to be made in order to avoid those potentially disastrous problems.
Other concepts for determining the ground velocity of an aircraft present certain disadvantages. They often rely on a relationship (e.g., cross-correlation) between signals transmitted and received by the aircraft. Thus, they require the aircraft to carry a transmitter, a transmitting antenna, and an energy source for supplying the transmission power. These components add undesirable bulk, weight, and hence expense to the aircraft navigation system. Because these systems depend upon a transmitted signal to bounce off an object and return to the aircraft before it can be processed, they experience substantial time delay in obtaining velocity data. Some such systems (for example, those utilizing the Doppler effect) must bounce the signals off of objects located ahead of the aircraft and thereby may encounter the problem of getting no reflected signal back to the aircraft. This problem is particularly acute when attempting to bounce the signal off of flat horizontal surfaces, such as bodies of water. Also, these active systems inherently possess additional complexity and offer greater potential for failure, thus presenting increased risks as well as problems and costs associated with maintenance and system redundancy. Furthermore, some of these systems utilize moving detectors or a multiplicity of detectors for ground speed determination, adding further complexity which reduces system reliability.
Present wind shear determination systems rely on an inertial reference or ground generators coupled to an elaborate computational network. The time required to present the wind shear data to the flight crew by these systems is considerably more than is desirable for safe flying conditions.